The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An optical image scanner typically includes scanning hardware (i.e., contact image sensors) that transfers data of a source image scanned by the optical image scanner to a processor and/or controller, wherein the processor and/or controller creates an image (a scanned image) based on the data. With the industry trend toward lowering hardware and manufacturing costs, optical image scanners often employ sensors having a lower signal-to-noise ratio (SNR), lighting that provides a lower output (e.g., dimmer lighting), and less expensive cabling (e.g., unshielded cabling).
Using both lower output lighting and sensors having a lower SNR to scan images commonly result in scanned images that include noise. This noise in a scanned image is often referred to as random noise, white noise, Gaussian noise, or the like. Low pass filtering is commonly used to reduce the level of noise in a scanned image. However, a disadvantage of this approach is that basic low pass filtering generally causes blurring of fine detail in images.
Many optical image scanners use a scanning mechanism driven by a motor in which a sensor or sensor array is sequentially moved or passed over adjacent lines of a source image. As the sensor moves across the source image, the sensor transmits data in the form of a serial sequence of scanned color and/or brightness values (referred to herein as “scanned values”). The scanned values are commonly analog values sampled and transmitted at a relatively high frequency. Because of design and packaging constraints, a processor and/or controller that controls the motor and processes the scanned values may be located at a significant distance from the scanning hardware. Coupled with the lack of cable shielding, this creates the potential for switching currents and voltages in nearby circuits, such as control signals for the motor, to distort portions of analog signals from the sensor before the portions of analog signals reach the processor and/or controller. This often gives rise to what is commonly referred to as “salt and pepper” noise, which appears as distributions of very bright or very dark pixels on portions of a scanned image.
Median filtering (e.g., a nonlinear digital filtering technique of replacing entries with a median of neighboring entries) is commonly used to reduce the salt and pepper noise. However, a disadvantage of this approach is that basic median filtering also causes blurring of fine detail in images and can even make fine lines go away.